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10,000-Year Study Finds Oceans Warming Fast, But From a Cool Baseline

By Andrew C. Revkin October 31, 2013 3:16 pmOctober 31, 2013 3:16 pm

I had a fascinating and fruitful chat with Yair Rosenthal of Rutgers and Braddock Linsley of Columbia University — two authors of an important new Science paper extracting 10,000 years of temperature changes in fairly deep Pacific Ocean waters from fossil plankton buried in the seabed off Indonesia.

The study finds that the rise in ocean heat (and temperature) in recent decades is far faster than anything seen earlier in the Holocene, the period since the end of the last ice age. But the researchers say that this rise is from a relatively cool baseline. Between 10,000 and 8,000 years ago, at depths between 500 and 1,000 meters, the Pacific Ocean was some 2 degrees Celsius (3.6 degrees Fahrenheit) warmer than today for many centuries.

In our chat, Rosenthal said the work reveals the enormous capacity of the oceans to act as an efficient and copious reservoir for heat: “Maybe the ocean is taking the heat more and won’t exhale it as much.”*

We may have underestimated the efficiency of the oceans as a storehouse for heat and energy…. It may buy us some time – how much time, I don’t really know – to come to terms with climate change. But it’s not going to stop climate change.

Things are more interconnected, I think, than we thought. We can’t think of these as just European events or Northern Hemisphere events. We’re in the middle of the warm pool in the western Pacific on the Equator or south of the Equator and still we’re seeing these century-scale events – the medieval warm period and the little ice age. The Holocene thermal maximum 10,000 years ago was [also] thought to be the Northern Hemisphere at first. I think these events are global and we would expect other events to be, as well.

I’m sorry that I don’t have a full transcript of my video chat with the authors. This will unavoidably frustrate some readers, but I simply don’t have the time given my responsibilities in teaching and other work besides this blog.

A recent slowdown in global warming has led some skeptics to renew their claims that industrial carbon emissions are not causing a century-long rise in Earth’s surface temperatures. But rather than letting humans off the hook, a new study in the leading journal Science adds support to the idea that the oceans are taking up some of the excess heat, at least for the moment. In a reconstruction of Pacific Ocean temperatures in the last 10,000 years, researchers have found that its middle depths have warmed 15 times faster in the last 60 years than they did during apparent natural warming cycles in the previous 10,000.

“We’re experimenting by putting all this heat in the ocean without quite knowing how it’s going to come back out and affect climate,” said study coauthor Braddock Linsley, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory. “It’s not so much the magnitude of the change, but the rate of change.”

In its latest report, released in September, the UN’s Intergovernmental Panel on Climate Change (IPCC) noted the recent slowdown in the rate of global warming. While global temperatures rose by about one-fifth of a degree Fahrenheit per decade from the 1950s through 1990s, warming slowed to just half that rate after the record hot year of 1998. The IPCC has attributed the pause to natural climate fluctuations caused by volcanic eruptions, changes in solar intensity, and the movement of heat through the ocean. Many scientists note that 1998 was an exceptionally hot year even by modern standards, and so any average rise using it as a starting point would downplay the longer-term warming trend.

The IPCC scientists agree that much of the heat that humans have put into the atmosphere since the 1970s through greenhouse gas emissions probably has been absorbed by the ocean. However, the findings in Science put this idea into a long-term context, and suggest that the oceans may be storing even more of the effects of human emissions than scientists have so far realized. “We may have underestimated the efficiency of the oceans as a storehouse for heat and energy,” said study lead author, Yair Rosenthal, a climate scientist at Rutgers University. “It may buy us some time – how much time, I don’t really know. But it’s not going to stop climate change.”

Ocean heat is typically measured from buoys dispersed throughout the ocean, and with instruments lowered from ships, with reliable records at least in some places going back to the 1960s. To look back farther in time, scientists have developed ways to analyze the chemistry of ancient marine life to reconstruct the climates in which they lived. In a 2003 expedition to Indonesia, the researchers collected cores of sediment from the seas where water from the Pacific flows into the Indian Ocean. By measuring the levels of magnesium to calcium in the shells of Hyalinea balthica, a one-celled organism buried in those sediments, the researchers estimated the temperature of the middle-depth waters where H. Balthica lived, from about 1,500 to 3,000 feet down. The temperature record there reflects middle-depth temperatures throughout the western Pacific, the researchers say, since the waters around Indonesia originate from the mid-depths of the North and South Pacific. [Click to read the rest, including input from Kevin Trenberth of the National Center for Atmospheric Research.]

Updated, Nov. 1, 8:30 a.m. | * The quotation at the asterisk above was added after I had time to do a little transcribing. Here’s another core point in the video:

Linsley: I think this shows we need to focus some more attention on the places in the northern and southern hemispheres where the deep ocean is talking to the atmosphere and absorbing this heat and I think we need to spend some more time to understand how that water makes its way towards the Equator. We’re essentially running a large experiment where we’re putting this heat into the deep ocean and we don’t quite know what the downstream effects are going to be. There could be positive effects, buffering effects, there could be some pretty big negative effects.

Revkin: In a way, doesn’t the early Holocene provide at least a hint of what that might look like?

Linsley: Our results would suggest that there was more heat in the oceans in the early Holocene but it absorbed that heat much more slowly than it is now, when there are much more rapid changes going on.

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.